Abalyaeva V.V.*, EfimovO.N.*,GusevA.L.**
*Institute of Problems of Chemical Physics RAS, 142432, Moscow region, Chernogolovka **Russian Federal Nuclear Centre - All-Russian Research Institute of Experimental Physics, Sarov
Тел.: (096) 522-18-87, факс: (096) 517-89-10, e-mail: efimov@icp.ac.ru (Efimov O. N)
HYDROGEN DETECTION USING Pd-POLYANILINE/POLYVINYL ALCOHOL -PHOSPHORIC ACID/GLASSY CARBON ELECTROCHEMICAL SYSTEM
It has been shown that amperometric sensor involving Pd catalyst immobilized in the polyaniline matrix and polyvinyl alcohol- phosphoric acid polymer electrolyte can be used in hydrogen concentration measurements.
1. INTRODUCTION
Because of a rapid development of hydrogen power engineering, an urgent problem is hydrogen detection and monitoring in closed shells on its storage either in liquid or adsorbed state. Though there are many conventional hydrogen sensors, the interest to the design of efficient easy operating devices is steadily growing. Such devices are electrochemical sensors based on hydrogen interaction with noble metals (Pd and Pt) [1]. Catalytic dissociation of hydrogen results either in changes in electrode potential (po-tentiometric sensor) or the appearance of current passing through an electrochemical cell (amperometric sensor). In the latter case anode-activated hydrogen finally reduces an appropriate oxidant (oxygen, organic and inorganic oxidants). The advantage of amperometric sensors consists in a linear hydrogen concentration dependency of current. Palladium as a thin foil and palladium coating on conducting supports (carbon, nickel grid) can be used as catalysts in hydrogen activation and oxidation. However, material is destructed because of high hydrogen solubility in palladium and parallel changes in volume. An appropriate procedure for the preparation of catalytically active anodes is palladium dispersion in conducting polymer matrices [2,3]. Earlier we developed the procedures to apply conducting polymer-polyaniline thin coatings on non-noble metals and simultaneously insert noble metal complexes [4,5]. The inclusion of metal in a conducting polymer matrix provides optimal conditions for metal to be in high dispersed state and a polymer matrix to provide high electronic and ionic conductivity. Hydrogen easily diffuses through the matrix due to its high porosity [6]. Moreover, the consumption of noble metal sharply decreases, and non-noble metal or glassy carbon supports can be used as contacts. Specific requirements to hydrogen sensors in cryogenic plants are high sensitivity at low temperatures and stable operation in vacuum. Therefore, we chose solid polymer electrolyte based on polyvinyl alcohol and phosphoric acid, which was described in [7].
2.EXPERIMENTAL
Polyaniline was prepared by the electrochemical oxidation of aniline by ammonium sulfate in dilute hydro-
chloric acid at ~0 °C 34 g of ammonium persulfate dissolved in 76 ml of water was added slowly to 10 ml of aniline dissolved in 300 ml of 1M HCl. The resulting precipitate was washed, treated with aqueous ammonium (0.12 mol/l) to convert to aniline base, dried in vacuum at 60-70°Q and then dissolved in concentrated formic acid (99.7 w/w%). When the insoluble share of polyaniline was removed by centrifugation, the solution was concentrated up to 30 g/l.
Electrochemical synthesis of polyaniline was performed in air in 0.4 М aniline sulfate in 0.1 М H2SO4 using a three-compartment electrochemical glass cell. The cathode and anode spaces were separated by a porous glass membrane. Electrochemical measurements were performed using a PI-50-1 potentiostat with a PR-8 programmer and the spectra were measured using a Specord UV-VIS spectrophotometer. The working electrode materials were glass covered by a transparent conducting In2O3-SnO2 layer, glassy carbon, Ti, and Ta. The working electrode surface was equal to 0,5-1 cm2. A glassy carbon plate of 2 cm2 area was used as a counter electrode, and Ag/AgCI was used as a reference electrode.
Fig.1. Cyclic voltammograms of the electrodes: (1) SnO2-In2O3 in PdCl2solution (2-10-3 mol/l) in 1M H2SO4; (2) the same electrode covered by PAn, 0.1 M H2SO4.Potential scan rate is 20 mV/s
AbaLyayeva V.V., Efimov O.N.,Gusev A.L.
Hydrogen detection using Pd-polyaniline/polyvinyl alcohol - phosphoric acid/glassy carbon electrochemical system
Electrochemical synthesis of polyaniline was performed at potential scanning in the -0.15 ■ +0.75 V range. The scan rate was 20-50 mV/s. Two techniques of palladium insertion in the polymer matrix were used. In the first one PdCl2 was added to the electrolyte. At positive potential scanning one observed the growth of polyaniline coating on the electrode surface, and the PdCl3-anion was involved as a counter ion in the positively charged polymer matrix. On reverse potential scanning to cathode potentials the salt was reduced in the polymer bulk to high dispersed metal. In the second case polyaniline base was dissolved in formic acid and mixed with a calculated amount of PdCl2 in 1 M H2SO4, and then was cast onto the working electrode surface. The dried electrode was then cycled in the -0.15 ^+0.75 V range in 0.1 M H2SO4. Solid electrolyte was prepared from the mixture of polyvinyl alcohol (PVA), H3PO4, and H2O. The measurements were performed in a temperature-controlled cell at temperatures from 10 up to 90°C. All the potentials were measured with respect to the AgCl reference electrode.
Fig. 2. Absorption spectra of PAn on SnO2-In2O3 electrod after keeping for 15 min at potentials:(V): (1) -0.2; (2) +0.35; (3) +0,7.Spectrum 4 -film (1) kept for 1 min in PdCl2 solution (2.10-3mol/l). Spectrum 5 -the same film in H atmosphere
3. RESULTS AND DISCUSSION
The rate of molecular hydrogen oxidation on the Pd catalyst immobilized in the polyaniline matrix and, therefore, the current value are limited by hydrogen diffusion to the catalyst and the following hydrogen catalytic ionization, electron transfer to a contact, and proton diffusion in electrolyte.
H2gas ^ [MeH2 « Me ■ 2H ] 2H + + 2e.
Thus, current increases with partial pressure of H2.
Investigation of polyaniline - PdCl2 system: electrochemical behavior and spectrophotometry
Redox processes in poyaniline are reversible in the -0.1 - +0.8 V potential range. In fully reduced (leuco em-eraldine, -0.1 - +0.1 V) and fully oxidized (pernigraniline, +0.5 - -0.8 V) states the polymer is a dielectric, and has
mixed electronic-ionic conductivity in an intermediate state (emeraldine, +0.1 - +0.5 V). The structure of poly-aniline can be presented by the formula
where Y depends on the oxidation state, and X is defined by the polymer chain length, which can consist of hundreds and thousands of repeated units. Polymer oxidation is accompanied by the addition of hydrogen to nitrogen atoms (Y decreases) and compensation of positive charge appearing in the polymer chain by anion (A_) intercalation in the polymer matrix. For emeraldine Y=0.5.
This process is similar to semiconductor doping and in terms of band theory corresponds to the formation of new energy levels inside the energy gap. The charged state has an unpaired spin and is delocalized over several polymer chain units. Such a combination is called po-laron, i.e. a quasi-particle, which is involved in charge transfer and provides electronic conductivity [2]. Ionic conductivity is due to diffusion of anions. Polyaniline can be reduced and oxidized either chemically (strong oxi-dants and reductants, strong acids) or electrochemically on an electrode in electrochemical cells.
Polyaniline does not react with hydrogen in the absence of catalyst. The cyclic voltammogram (CVA) of PdCl2 (Fig.1, CVA 1) shows that the reduction to Pd0 is observed at +0.3 V, and oxidation to Pd2+ is observed +0.6 V on reverse potential scanning to the anodic range. The potential difference of 0.3 V indicates that reduced Pd0 is present as adsorbed clusters on a working electrode. It should be noted in this potential range (+0.3 -+0.6 V) polyaniline is in electroconducting state (emer-aldine). This implies that the polymer matrix provides conditions for charge transport. Catalytic activation of hydrogen is accompanied by current growth, which is proportional to hydrogen concentration in the system. It is essential for chemical dispersion of palladium in the polymer matrix (2nd technique) that in its deprotonated and reduced form (leuco emeraldine) the polymer is itself a strong enough reductant to reduce Pd+2 to Pd0, which catalyzes activation of molecular hydrogen.
This conclusion was justified by spectrophotome-try of polyaniline film prepared electrochemically on In2O3 - SnO2. It is seen from Fig.2 that the oxidation state of polyaniline can be identified from different absorption in the visible spectral range. The polyaniline film fully reduced on the electrode at -0.2 V (leuco emeraldi-ne, Y>0.5) is almost colorless (Fig.2, spectrum 1) since it consists mainly of amine fragments and is a dielectric. On potential shift to +0.35 V (emeraldine, Y=0.5), the oxidized film turns green and absorption is observed at 400 and 800 nm (Fig.2, spectrum 2). The polymer has maximal conductivity in this state. Further oxidation results in the increased number of quinoid fragments in the polymer chain (Y<0,5), conductivity decreases, and the film turns blue. No absorption is observed at 400 nm and a wide band appears at 700 nm (Fig.2, spectrum 3). If a fully reduced film (Fig.2, spectrum 1) is immersed in PdCl2 solution, it immediately turns deep blue (Fig.2, spectrum 4). The spectra 3 and 4 are similar and correspond to pernigraniline. This is also observed at electro-
100 s
1 1 \\ u 1 1 1 1
\\ \
\\_ ^ — -
-0,2 0 +0,2 +0,4 +0,6
a)
0,1 mA
f 0 +0,2 +0,4 +0,6
b)
mA
J_L
0,2 0,4 0,6
E,V
c)
Fig. 3. Changes in electrode currents: (a) parent PAn, (b) PAn + PdCl2 in Ar atmosphere (- -) and Ar-H2 mixture (1:1) (—) at different potentials; (c) dependency of current on P
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